Semipermeable membranes are substantially permeable to a liquid and substantially impermeable to solutes based on the nature of their selective barrier. Osmotically driven membrane separation generally relies upon driving forces associated with the passage of draw solutes through one or more support layers of a membrane used in the separation process.
Polymeric membranes used in liquid separations are typically thin-film composite (TFC) membranes which generally include a selective barrier on a porous support structure. Recent development of highly selective membranes has been focused primarily on the reverse osmosis (RO) process. Reverse osmosis is a pressure driven process in which the primary resistance to water flux through the membrane is hydrodynamic once the osmotic pressure of the solution is overcome by an excess of hydraulic pressure. Forward osmosis (FO), by contrast, is a diffusion driven process. The factors affecting water flux in RO and FO processes are different, in turn requiring different membrane structures for optimum performance.